@Background Pony #6BDC  
I think a hypothetical fix can be pretty easy: you keep the “Demons cost health this turn” effect only, and set mana cost when you want this to come online, turn 5 or 6 probably (this is definitely a mid-game effect - it’s broken too early on and loses value too late when mana isn’t an issue anymore).  
Then you can try to use up all your remaining health to drop as many demons as possible finishing with a Jaraxxus or Mal’Ganis to seal yourself off.

There’s a reason why Cho’Gall only affects the next spell played, and not all of your spells and especially not minions, and more still why he costs 7. Paying health instead of mana is already an upside, the ability to dump a minion even as simple as Fearsome Doomguard on turn 2 (sometimes T1) is absolutely insane, beyond even double innervate shenanigans, +3/ +3 and charge itself is worth well over 3 mana per minion played - no matter how you cut it, this card is, as Maro would put it, baaah-roken.
 
There’s really no easy way to fix this - raising the cost defeats the purpose of the effect, and allowing the card to exist at such a low cost defeats a lot of design principles of Hearthstone. Look at Summoning Portal - it’s a 4 mana 0/4 and won’t lower the cost of minions below 1.
 
Your best bet, for a similar effect is something like:  
“The next Demon you play this turn costs (X) less. Give that minion +Y/ +Y and charge. Deal X damage to your hero.”  
And then pick some arbitrary fair numbers for X and Y, like 4 and 2 - though at this point you’re just giving Warlock a kinda-sorta Innervate, and the added buff and charge almost wholly negates the “cost” of health, so even then it’s still kinda busted. I’d argue to omit charge entirely since it’s not really a Warlock ability (save for Doomguard who comes at the additional cost of a whopping 2 discards).

To also mention. I think you underestimate using your health as a ressource. There’s a reason why Flame Imp is really good, its a 3/2 for 1 mana. Even if it costs 3 health to summon him. So, playing a 9/9 with Charge for 6 health? AMAZING VALUE.

@Ledvi  
This card is definitely an aggro-oriented card, it’s not a control-oriented card.  
Polymorph costs 4 mana. Mages cant deal with big threats on the first few turns, except freeze it and wait.  
There isnt a single class that can deal with more than one minion on turn 2. Having multiple big threats is an instant victory.
 
I dont know why someone would wait for Wilfred’s effect. People dont even play him because the effect is so bad, its not good enough for how bad a 4/4 on turn 6 is.
 
Also, Coin this then Jaraxxus gives you a infinite series of 6/6 infernals, and you pay the cost before Jaraxxus is summoned. It’s incredibly good.

@Chaoslux  
This card isn’t focusing on summoning demons on early game, but more focus on late game. Besides if you play this early, it would get wasted by being destroyed (Priest and Hunter), transmogrified (Mage), put back to your hand (Rogue). But then again, doing so will make you suffer as well since you’re summoning bigger demons.
 
If I have two of these in my hands, I would save it for Wilfred Fizzlebang. Thanks to Wilfred’s effect, 0 mana means 0 health loss.

@Animegx43  
Nope. Jaraxxus won’t be affected by this card since it cost 9 to summon him. Even if I made this card 1 mana cost, summoning Jaraxxus would waste this card since you become the Eredar Lord.

Ive seen some overpowered cards in my life, but JESUS is this card broken. It could simply say “Demons costs health this turn” and it would be a pretty good card. The fact you needed to add a free +3/+3 and Charge buff makes it straight up ridiculous.  
Fearless Doomguard and Infernals in opening hand makes this a OTK on turn 3. Even if it wasn’t a kill, people are currently up in arm about how Shamans are overpowered for having a 4mana 7/7, and you’re giving warlocks multiple 9/9 for 3 mana?
 
Edit: OH MY GOD! It’s actually 2 mana. That’s even worse. This means you can literally coin several 9/9 minions with Charge on your first turn.

So, Jarraxus on turn 2? Fun.

Fun fact: “Balrog’s Blood” is what they call the drink “Sex on the Beach” in Endor.